Not Applicable.
This invention relates generally to semiconductor lasers and more particularly to semiconductor lasers that emit radiation in the mid-infrared region.
As is known in the art, semiconductor lasers typically include a plurality of layers of semiconductor materials. A conventional semiconductor laser includes a first semiconductor material corresponding to an active core in which light can be generated or can otherwise propagate. First and second cladding layers also provided from semiconductor materials are disposed about the active core. The first and second cladding layers are provided having a lower index of refraction than the active core, thus causing light to be generally contained within the active core.
The active core can be comprised of a plurality of active structures in combination with associated confinement layers. The confinement layers are provided surrounding each of the one or more active structures. The confinement layers are also referred to as absorber layers.
An xe2x80x9celectrically pumpedxe2x80x9d laser is a semiconductor laser in which light can be generated in and emitted from the active core in response to an electric current provided to the semiconductor laser. When the first cladding layer is doped to provide an n-type characteristic and the second cladding layer is doped to provide a p-type characteristic, the two cladding layers taken together with the active core form a diode. When the diode is forward biased with an external power supply, an electric current is generated through the semiconductor laser, and electrons and holes recombine in the active core resulting in light that propagates in and is emitted from the active core.
Alternatively, an xe2x80x9coptically pumpedxe2x80x9d laser is a semiconductor laser in which light can be propagated in and emitted from the active core in response to light from an external light source, properly directed and at the proper wavelength. With this particular arrangement the cladding layers can be provided without the need for dopants.
It is recognized that semiconductor lasers can be constructed to be either electrically pumped or optically pumped and to have either emission of light from the edge of the semiconductor laser or from the surface of the semiconductor laser. It will also be appreciated that conventional cladding layers that contain high fractions of Aluminum (Al) are generally difficult to dope to provide the first and second cladding layers with the n-type and the p-type characteristics respectively. Also, conventional cladding layers that contain high fractions of Aluminum (Al) tend to be susceptible to degradation due to oxidation.
A wide variety of conventional semiconductor lasers have been described. In general, the physical characteristics of the semiconductor lasers are determined in accordance with a variety of factors, including but not limited to the desired wavelength of the laser light, the desired output power, the desired divergence angle, the desire for the electrically pumped or the optically pumped semiconductor laser, and the desire for edge or surface emission.
In one embodiment, a semiconductor laser made to operate at the mid-infrared region, i.e. having light having a wavelength of between 1.5 and 12 micrometers, has an active core consisting of GaSb in combination with other elements, and first and second cladding layers that include Aluminum. Such semiconductor lasers typically have an active core with an index of refraction of approximately 3.8, and first and second cladding layers with an index of refraction of approximately 3.2.
As described above, it is understood that it is desirable to provide the first and second cladding layers having a lower index of refraction than the active core in order to confine light within the active core. It is also understood that, if the index of refraction of the first and second cladding layers is greatly different from that of the active core, the light that emerges from the active core will have a wide divergence angle. A semiconductor laser for which the active core has an index of refraction of approximately 3.8 and the first and second cladding layers have an index of refraction of approximately 3.2 conventionally provides a divergence angle of nearly ninety degrees in the plane perpendicular to the plane of the epilayers.
Since it is difficult to collect all of the light output power from a semiconductor laser having a relatively wide divergence angle, a narrow divergence angle is preferred in most applications.
It would, therefore, be desirable to provide a semiconductor laser that operates in the mid-infrared region and for which the divergence angle is relatively narrow. It would also be desirable to provide a semiconductor laser that operates in the mid-infrared region and that contains little or no Aluminum in the first and the second cladding layers. It would be still further desirable to provide a semiconductor laser that operates in the mid-infrared region and for which the index of refraction of the first and second cladding layers can be adjusted so that it can be matched with the index of refraction of a variety of types of active cores.
The present invention provides a semiconductor laser that operates in the mid-infrared region and that provides a relatively narrow divergence angle. While a particular active core is described, it will be understood that this invention applies equally to a variety of active cores.
In accordance with the present invention, a semiconductor laser includes an active core and cladding layers on each side of the active core, wherein at least one of the cladding layers consists essentially of GaSb. This structure provides a semiconductor laser that operates in the mid-infrared region of 1.5 to twelve microns. The active core can be comprised of In, Ga, Sb and As.
With this particular arrangement, the cladding layers provide an index of refraction less than but close to the index of refraction of the active core, thus providing laser light emission having a narrow divergence angle. Furthermore, cladding layers that contain no Aluminum (Al) are more easily doped to provide cladding layers having n-type and p-type characteristics and have smaller bandgap discontinuities. This facilitates the fabrication of an electrically pumped semiconductor laser.
In accordance with another aspect of the present invention, a semiconductor laser includes an active core and cladding layers on each side of the active core, wherein at least one of the cladding layers consists essentially of AlxGa1-xAsySb1-y, wherein the Al fraction, x, is selectable in the range of zero to twenty five percent. This structure provides a semiconductor laser that operates in the mid-infrared region of three to twelve microns.
With this particular arrangement, at least one selectable cladding layer provides an index of refraction less than but close to the index of refraction of a variety of types of active cores, thus providing laser light emission having a narrow divergence angle.